The chemical effects of ultrasound have been studied in relation to sonodynamic cancer therapy. Recent in vivo and in vitro studies by Umemura et al (Jpn. J. Cancer Res. 81, 962, 1990) have indicated antitumor effects of hematoporphyrin activated by ultrasound. A suggested mechanism involves the sonochemical generation of singlet oxygen. One of the proposed methods of testing for singlet oxygen is the conversion of a sterically hindered cyclic secondary amine (2.2.6.6 tetramethyl-4-piperidone. TMPone) to the corresponding nitroxide which can be detected by electron spin resonance. A detailed study of the sonolysis and gammaradiolysis of TMPone showed that TMPone cannot be used for the detection of singlet oxygen in sonochemistry since the same nitroxide is also produced by the reaction of hydroxyl radicals in the presence of oxygen with a similar Ph dependence. Our recent studies have shown that transient cavitation (that is, the formation, growth and violent collapse of gas bubbles) occurs in argon-saturated aqueous solutions exposed to typical diagnostic ultrasound pulses from a Ultramark-9 ultrasound system. The 2.25 Mhz ultrasound pulses had a pulse width of 0.8 microsec, a pulse repetition frequency of 50 Khz and a spatial peak pulse average intensity of 100 watts/cm2. Hydroxyl radical formation was detected by means of a highly sensitive assay using high pressure liquid chromatography with electrochemical detection of picomoles of the hydroxylation products of salicylate.